How do I move the dock to the left of my external display?

You cannot.

You cannot position the dock where two adjacent screens overlap.

Since the left side of your external display aligns with the right side of your internal display, this area cannot be used for the dock (either the left side of the external, nor the right side of the built-in).

When you choose Left or Right, the dock is positioned based upon the concept of a single, unified desktop (i.e. 'Right' will always appear at the right hand side of the right-most screen, and 'Left' will appear on the left hand side of the left-most screen). At no point will the dock appear in the middle of the global desktop.

Hello kdemelle,

Thank you for posting to Apple Support Communities.

We understand you'd like to place your dock on the left-hand side of your external display. You should be able to do this if you set the external display as your main display at Apple menu () > System Preferences > Displays > Display Settings

See here: Change Displays preferences on Mac

We hope that helps. Take care.

Hey there kdemelle,

We tested this and the with the Dock set to the left, it goes to whichever display is on the left, no matter what display is set as the main one. If you like, you can leave feedback about this here:

Product Feedback - Apple

Take care.